1. Field of the Invention
The present invention relates to a vacuum dry pump for evacuating a reaction chamber of a semiconductor-manufacturing device and the like or for discharging gaseous material generated in the reaction chamber.
2. Prior Art Description
The roots pump is one type of vacuum dry pumps and widely used as a booster pump since it has a characteristic that, in a low operating pressure, its discharging speed is higher while its power requirement is smaller. As shown in FIG. 6, for example, a multistage pump system comprising a roots pump 100 installed at vacuum side and an oil rotary pump or dry pump 200 installed at ambient side is used for evacuating the reaction chamber of a semiconductor-manufacturing device and the like to make a high vacuum condition therein.
FIG. 7 shows a typical characteristic curve of discharging speed of the roots pump and that of the oil rotary pump or dry pump. As is shown by these characteristic curves, the discharging speed of the roots pump is higher than that of the oil rotary pump or dry pump which is an ambient-side primary pump. Due to such difference of the discharging speeds in the system of FIG. 6, an over pressure condition may occur in the roots pump, which increases the power requirement of the roots pump when its operating pressure is near the ambient condition. To reduce the power requirement, in such case, the roots pump should be stopped or its rotational speed should be lowered.
It is preferable that the property of the power requirement of the pump and/or the pump system should be almost flat even if their suction pressure varies. However, according to the characteristics of the power requirement of the oil rotary pump or the dry pump that have conventionally been used with the roots pump, their power requirement also tends to become large with increasing of the suction pressure toward the ambient condition. Therefore, it is also impossible to suppress their power requirement change.
On the other hand, if the conventional multistage pump system of FIG. 6 is used to obtain a high compression ratio (a high degree of vacuum), the conventional system has another problem that it requires a large space for installation and cannot be made compact.
A main object of the present invention is, taking the aforementioned problems into consideration, to provide a composite dry vacuum pump wherein the power requirement change is small and the structure is so compact.